PROJECT SUMMARY Traumatic brain injury (TBI) is the leading cause of acquired neurologic disability in children; yet, no effective therapies exist. Secondary injury from inflammation increases neurologic disability from damage after impact. Microglia (the brain's resident immune cells) mediate inflammatory release of reactive oxygen species (ROS) and cytotoxic factors early after TBI. Microglia that become reparatory remove debris by phagocytosis and resolve inflammation. The immature brain is highly vulnerable to secondary injury due to its low antioxidant reserve and vigorous inflammatory response. Agents that move microglia away from inflammatory towards reparatory activity may decrease neurologic disability after TBI, particularly in children. Docosahexaenoic Acid, or DHA, is a candidate therapy for childhood TBI. DHA is a fatty acid essential for normal brain growth and function that has antioxidant and anti-inflammatory activity. In rats, DHA restores brain DHA losses after TBI. In cultured microglia, DHA promotes reparatory activity. Using our pediatric TBI model, controlled cortical impact (CCI) in male 17-day old (P17) rats, DHA diet before CCI reduced learning deficits (Morris Water Maze, MWM) brain lesion volume and white matter injury (histology and MRI, Magnetic Resonance Imaging). We also showed that DHA given after CCI improved rat pup outcomes. Intraperitoneal (IP) DHA at 30 minutes after CCI, followed by up to 60 days of DHA diet, decreased ROS, microglial inflammatory genes at post injury day 7(PID7) and memory dysfunction (Novel Object Recognition, NOR). Whether DHA affects brain inflammatory markers in female pups, or if DHA affects microglial activity (ROS production and phagocytosis) after TBI in either sex, is unknown. Similarly, whether delayed (later than 30 min after CCI) or short (7-day course) DHA will retain acute and chronic neuroprotection, is not known. Finally, little is known about how DHA may modulate microglial activation after TBI. We hypothesize that either DHA at 30 min or 3h after CCI will decrease PID7 microglial inflammation in males and that only long term DHA will improve PID45-60 outcomes in both sexes. We hypothesize that DHA will decrease PID7 inflammation associated with decreased TLR4 and activated STAT1 proteins on cell membranes and caspase activity. We will use male and female P17 rat pups and CCI or sham surgery. We will compare onset of DHA (30 min or 3h) and duration (7 days to 60 days) to control. During week 1 after CCI, we will use cell membrane assays, tissue and microglial protein markers, and microglial activity to assess inflammation. From PID 45 to 60 we will use NOR/ MWM testing, histology and MRI for functional, anatomic and inflammatory outcomes. This proposal is significant because it focuses on the immature brain after severe TBI, a devastating condition for which no effective therapies exist. It will yield new knowledge on the effects of sex and DHA on microglial function after TBI and on optimal DHA initiation and duration timing. Results will enable future clinical trials of DHA with the potential to decrease the burden of pediatric acquired neurologic injury after TBI.